Pluto is a dwarf planet that lies in the Kuiper Belt at the edge of our solar system.

In the cover image you can see Pluto taken by New Horizons on July 14, 2015, from a range of 22,025 miles (35,445) kilometers. Credit: NASA/Johns Hopkins University Applied Physics Laboratory.

The striking features on Pluto are clearly visible, including the bright expanse of Pluto’s icy, nitrogen-and-methane.

Pluto’s image taken by New Horizons on July 14, 2015, from a range of 22,025 miles (35,445) kilometers. Credit: NASA

Pluto a dwarf planet

For a decade, Pluto was the subject of dispute, mostly because of its small size at 2,390 kilometers in diameter.

Finally, on August 24, 2006 Pluto lost its planet status, during the XXVI General Assembly of the International Astronomical Union, when most of the participants decided to declare it a “dwarf planet“.

Pluto is smaller even than the Moon, which has a diameter of 3,480 km.

Planets sizes
Relatives sizes of the Planets. Credit: web “”.

In addition, Pluto describes an unorthodox orbit around the Sun, since its inclination is not parallel to that of the Earth and those of the other seven planets of the Solar System.

But the main drawback for Pluto not being considered a planet is that its gravity is insufficient to clear the environment of its orbit.

Discovery of Pluto

The discovery of Pluto was no accident.

From an early age, Clyde William Tombaugh (1906-1997), who lived in Streator, a small town in Illinois, USA, became interested in astronomy.

In 1928, young Tombaugh built his own nine-inch telescope, using scrap machinery. He sent a few of his meticulous observations of various planets to the Lowell Observatory.

Very soon, in 1929, he was hired at said Observatory, as a junior astronomer, with the mission of finding a planet that should be beyond Neptune, as calculated by none other than Percival Lowell himself.

Tombaugh took pictures of portions of the sky and began searching until, in January 1930, after a year of searching, he found a change in the position of a 17-magnitude object, whose behavior mirrored that predicted for a trans-Nepturian planet.

Clyde Tombaugh
Clyde Tombaugh at his family’s farm with his homemade telescope (1928). Credit: Wikipedia. Web “”.

The discovery was confirmed by other observers and the new object became known to the world, as the ninth planet in the solar system.

This ninth planet was given the name Pluto, at the suggestion of Venetia Burney, an 11-year-old girl, a student at Oxford and fond of classical mythology and astronomy.

Venetia Burney
Venetia Burney, aged 11. Credit: Wikipedia. Web “”

She thought that Pluto, the name of the god of darkness, was very appropriate for this world that was supposed to be cold and dark.

The name passed by word of mouth, until it reached the ears of astronomers in the United States.

Lowell Observatory had the right to name the new discovered object. For this they received more than 1,000 suggestions of names.

Finally, each member of the Lowell Observatory was able to choose their favorite name from a short list of three names: Minerva, Cronos, and Pluto. Venice Burney received £ 5 as a prize for the choice of her proposed name.

On May 1, 1930, the new planet was named Pluto.

Soon after, this name of the planet served to call Pluto to one of Walt Disney’s characters and plutonium to a new chemical element.

Since then Pluto has been very photographed

The new and detailed images captured by the ‘Hubble’ space telescope show the true color of Pluto.

Although these are the most detailed photographs available so far, they do not allow us to see if there are craters or mountains on Plato’s surface, but they do allow us to observe the great variety of colors on their surface.

The Hubble Space Telescope has produced this fuzzy view of Pluto. Credit: NASA.

Pluto’s flushing has been particularly significant during the 2000-2002 period.

The photographs reflect how the frozen nitrogen becomes brighter in the north and darker in the south.

These changes appear to be the result of melting ice at the sunny pole and freezing at the other pole.

The changes occur on the planet’s icy surface when a new phase of its orbit begins.

Pluto orbits

The surface temperature is 233º C below zero. Pluto takes 248 years to go one revolution around the Sun.

Pluto orbit
What is there beyond Pluto’s orbit in our Solar System? Credit: web “”

Pluto’s reduced size is compounded by the fact that its orbit is very far from the orbital plane of the other 8 planets. Pluto takes 248 years to orbit the Sun.

Moons of Pluto

In 1978 it was discovered that a large moon orbited around Pluto: it was named Charon.

In 2005 two more moons appeared: Nix and Hydra. So far three moons or satellites of Pluto were known early 21st Century: Charon, Hydra and Nix.

On July 20, 2011, another moon was discovered, and a year later, on July 11, 2012, the discovery of Pluto’s fifth moon was announced.

Pluto satellites
Pluto satellites. Credit: web “”

The largest moon is Charon, which is 1,192 kilometers in diameter and 19,640 kilometers from Pluto. It was discovered in 1978.

Due to its size, similar to that of Pluto, it is considered a candidate to be classified as a dwarf planet or as a double planet, since their masses are similar.

Pluto is 7 times more massive than Charon and both orbit around the center of gravity outside Pluto, but closer to it.

The rotation of this pair is unique in the Solar System.

For now, Charon is still a moon of Pluto.

Over time, gravity has slowed the rotations of Charon and Pluto. Currently, they always present the same face each other.

The moons Hydra and Nix were observed for the first time in May 2005.

In 2006, Hubble Space Telescope confirmed the discovery of these two satellites.

It seems that both bodies orbit in the same plane as Charon and at distances two and three times greater.

Their orbits are very outer, so they are satellites of the Pluto-Charon system. Both appear to be between 100 and 150 km in diameter.

Pluto Earth comparison

Pluto size Earth
Comparison of Pluto-Charon and Earth-Moon. Credit: NASA, JHUAPL, SWRI, Gregory H. Revera
  • Diameter of the Earth: 12,742 km
  • Diameter of the Moon: 3,480 km.
  • Pluto’s diameter: 2,390 km
  • Charon’s diameter: 1,192 km

It is observed that the size of Pluto is only 2/3 the size of the Moon.


Plutoid is any object that meets the definition of a dwarf planet and that is at a greater distance than Neptune, with sufficient mass for its own gravity to overcome the forces of a rigid body in such a way that it acquires an almost spherical shape and does not it has emptied its orbit of neighboring bodies.

The dwarf planet Ceres is not a plutoid as it is located in the asteroid belt, between Mars and Jupiter.

Eris was considered for a time the 10th planet in the solar system. It is the largest plutoid ever discovered and is somewhat larger than Pluto.

It has a natural satellite called Dysnomia.

The other two objects recognized as plutoids are: Makemake and Haumea.

Plutoids. Credit: web “”

All other objects that orbit the Sun, beyond Neptune, are collectively considered minor bodies in the Solar System. It could be several thousand.